Decayed premolar tooth. Close up of a decayed premolar tooth in a 67-year-old man's upper jaw. Dental caries is the decay and crumbling of the teeth. It is caused by the bacteria in plaque, a substance which covers the surface of unclean teeth. The bacteria break down sugar in the mouth, producing an acid which progressively destroys the teeth. In severe cases such as this, root canal surgery or tooth extraction is needed. This decayed tooth is about to be extracted. For this view after the extraction, see image C029/2194.